The subject matter disclosed herein relates generally to deceleration of wind turbines.
Wind turbines are increasingly gaining importance as renewable sources of energy generation and are increasingly being applied to large-scale power generation applications. Maximizing energy output while minimizing loads of the wind turbines in varied wind conditions is a challenge that exists in harnessing wind energy.
A wind turbine typically includes a tower and a rotor rotatably coupled to two or more blades. The blades are acted upon by a wind flow to rotate the rotor. The speed of the wind turbine is dependent upon multiple factors including angle of attack, speed of wind, and pitch angle of a blade, for example. The angle of attack is an angle between a reference line of an airfoil of the blade and a direction of the wind flow acting upon the blade. The pitch angle of a blade of a wind turbine refers to a position of the blade with respect to the direction of the wind through which the blade rotates. The pitch angle of the blade may be changed to change the angle of attack of the blade and thereby change the speed of the wind turbine.
Speed control and emergency braking capability are important for maintaining structural stability and longevity of components within wind turbines. For example, controlling the speed of a wind turbine rotor below its maximum limit is important in order to avoid the damaging effects of high velocity winds on the wind turbine blades, rotor, and tower.
Under normal wind and operating conditions, deceleration of a wind turbine may be achieved by pitching the wind turbine blades to be more closely aligned with the direction of the wind. Under certain transient or fault conditions, blade pitching may be inadvisable or may provide an insufficient level of deceleration.
One example of a technique other than blade pitching which has been used for braking wind turbines during emergencies or other events that require stopping turbines, is mechanical braking. However, true dynamic control is difficult to achieve in mechanical braking during operation of wind turbines. Additionally, the amount of torque provided will typically not be constant over time due to variables such as temperature and wear.
In another technique other than blade pitching for braking, an electrical brake is used instead of or in combination with a mechanical brake. For example, to enable dynamic braking control, commonly assigned Schramm U.S. Pat. No. 8,080,891 describes an embodiment wherein a mechanical brake and an electrical braking circuit are activated simultaneously in response to a braking event such that the electrical circuit enables early and controlled absorption of excess power during a response delay of the mechanical brake.
In wind turbine embodiments wherein multiple brakes are available, it would be desirable to have a flexible control method for both braking and deceleration. Additionally, for embodiments including a mechanical brake, it would be desirable to have a method with a reduced duty cycle of the mechanical brake to enable longer life of the mechanical brake.